Advance angle is the angle between the pitch link attachment on the swash plate and the longitudinal axis of the blade.
Advance angle is the angle between the chord line and the pitch operating arm.
Advance angle is the angle between the pitch operating arm and the swash plate.
Advance angle is the angle between the advancing and retreating blades.
The compensation for phase lag can be built into the control run as an advance angle, meaning that control input is made to occur "in advance" of the desired effect by having the input point on a pitch change arm ahead of the blade. Thus, Advance Angle is the angle between the pitch link attachment on the swash plate and the longitudinal axis of the blade.
The size of the advance angle depends on how well the blade responds to flapping. On most two-bladed rotor systems, especially those with stabiliser bars, the pitch change arm leads the blade by 90°. Control inputs are initially fed to the stabiliser bar, which is at 90° to the blades, so the pitch change arm gets its input from the bar. When the swash plate is tilted forward, minimum pitch is applied to the bar when it is pointing forward (with the blades athwartships), so minimum pitch is being applied 90° before maximum flap down is needed.
On multi-bladed designs, the Advance Angle will be less than 90º. However, when the Advance Angle is less than 90º, it cannot fully compensate completely for phase lag. In this case, the difference between the Advance Angle and phase lag is made up by adjusting the control rigging.
Advance angle can also be used to correct for Inflow Roll (later) in forward flight.
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